Ether-type co-solvents of excellence for aqueous base-free Pt-catalyzed oxidation of concentrated 5-(hydroxymethyl)furfural to 2,5-furandicarboxylic acid

The biomass-derived 5-(hydroxymethyl)furfural (5-HMF) is readily converted in water to 2,5-furandicarboxyl acid (FDCA) that is well known as a renewable diacid monomer. However, the limited solubility of FDCA in water posed serious challenges to process development. Herein, the base-free Pt-catalyzed oxidation of 5-HMF was explored in aqueous binary solvent systems containing organic co-solvents such as dimethyl sulfoxide, N,N-dimethylformamide, gamma-valerolactone, dioxane, diethylene glycol dimethyl ether, and 1,2-dimethoxyethane (DME). The FDCA solubility and 5-HMF oxidation performance were investigated in co-solvent/H2O systems with different co-solvent fractions. When correlated with the solvatochromic parameters of co-solvents, the FDCA yield appeared to be in harmony with their polarizability and hydrogen bonding acceptance ability. Thus, infrared spectra were collected with three systems such as co-solvent/H2O, 5-HMF in co-solvent, and 5-HMF in co-solvent/H2O. The employed co-solvent was found to have different abilities to interact with the O-H and C=O moieties of 5-HMF via H-bonding and dipole-induced dipole attraction (related with the polarizability), respectively. Consequently, a high FDCA yield was achieved using three ether-type co-solvents in which these interactions are relatively weak. In a continuous-flow pattern for the base-free oxidation of 3 wt% 5-HMF solution, a FDCA yield of >98 % was achieved with 60 wt% DME/H2O under the optimized conditions.